1. Field of the Invention
This invention relates to electrostatic spray coating systems, and more particularly to a device for collecting free ions in an electrostatic spray coating system.
2. Description of the Prior Art
In electrostatic spray coating systems, the coating material is pumped from a supply to one or more spray guns which spray the coating material onto a part to be coated. The coating material may be either in the form of dry particles conveyed in a fluidized air stream or in the form of liquid atomized by the gun. The spray guns may charge the coating particles by means of a high voltage charging electrode. When the coating particles are sprayed from the front of the gun, they are electrostatically attracted to the part to be coated which is generally electrically grounded and suspended from an overhead conveyer or otherwise conveyed through a spray booth. The spray guns are mounted in the spray booth, either in a stationary position or on a reciprocator or other device which allows the gun to be automatically moved in a predetermined path. Once these charged coating particles are deposited onto the part, they adhere there by electrostatic attraction until they are conveyed into an oven where they are cured, or, in the case of powder coating, melted to flow together to form a continuous coating on the product.
An ion collector or counter-electrode has been found to be useful in attracting free ions which would otherwise flow from the gun to the ground part. When charging the powder, a large number of free ions are also generated. Without an ion collector, the free ions are conveyed with the coating material onto the part. This causes a charge to accumulate on the deposited coating layer until the local electric field strength is great enough to cause ionization within the coating layer. This "back-ionization" disturbs the deposited coating and results in craters and other defects in the cured coating. Back-ionization causes a dramatic reduction in transfer efficiency and has a detrimental effect on economic and environmental effectiveness of the powder coating process. By using an ion collector, these free ions are collected before they reach the part, the transfer efficiency is improved, and the appearance of the surface finish on the part can be improved.
An example of an ion collector is the anti-back-ionization (ABI) device shown in U.S. Pat. application Ser. No. 08/959,723 assigned to the assignee of the present invention. This patent shows an ABI probe which provides satisfactory free ion collection. The effective probe length is adjustable, allowing the position of the end of the probe relative to the electrode to be changed, so that the probe position relative to the electrode can be adjusted.
Another example of an ion collector is shown in U.S. Pat. No. 4,921,172, issued to Belmain et al., in the form of a counter-electrode mounted on a powder spray gun on the front of the gun. Yet another example of an ion collector is shown in European Patent Publication No. 0,620,045 in the form of a counter-electrode ring fixedly mounted around the front of the gun. These ion collectors are fixed in position or built into the gun, so that they do not provide easy adjustment or removability.
The presence of an ion collector or ABI device or probe allows users to dramatically reduce the field strength between the gun and the grounded part as well as significantly reduce the free ion current to the part. Therefore, the development of back ionization is reduced or delayed, and the strength of the electric field between the gun and part is reduced. The reduction in field strength results in improved penetration of recessed areas or Faraday cage areas on the part. Positioning the ABI probe, however, is very important in fully realizing the positive effects of the probe. If the ABI probe is positioned properly, the coating results are comparable to those achieved using statically charged coating systems, and the ease of application is similar.
Maintaining the proper distance between the tip of the ABI probe and the part, however is more difficult than might first be contemplated. The distance between the gun and the part changes based upon the geometry of the part and upon changes in the parts assortment. Many parts include recessed areas which are significantly farther from the gun than the remainder of the part, and many parts are presented on production lines mixed with other parts which have significantly different shapes. As the distance between the gun and part changes, the positioning of the ABI probe must be manually adjusted to provide for maximum positive effect of using the ABI probe as an ion collector. Unfortunately, manual re-positioning is usually not possible in the middle of a production run as different shaped parts enter the spray booth. While manual re-positioning can be performed periodically taking into account the general shape and geometry of the parts on a specific production run, such manual re-positioning of the ABI probe can be rather cumbersome and, as a result, tends to be done rarely in many production applications.